1. Field of the Invention
The present invention relates to a printed circuit board. Particularly, the present invention relates to a printed circuit board for a high speed processing, in which a plurality of both-face copper clad laminates with circuits formed thereon, and a plurality of clad laminates with circuits not formed thereon are alternately stacked together, whereby impedance defects due to the non-uniformity in the inter-layer insulating distances are eliminated.
2. Description of the Prior Art
Recently, there has been developed the RAMBUS DRAM which shows a transmission rate 10 times as fast as that of the existing high speed synchronous DRAM. This RAMBUS DRAM is mostly used in electronic apparatuses requiring a large capacity graphic memory such as work stations, personal computers, digital televisions and the like. In this RAMBUS DRAM, the bank interleaving function is adopted, and therefore, the data processing capability is drastically improved. Thus it is expected that this RAMBUS DRAM will be used as the main memory of the personal computer as well as the graphic memory.
This RAMBUS DRAM shows not only a high operating speed compared with other kinds of DRAM, but also a large transmission rate. For example, the 144M RAMBUS DRAM which is the concurrent version shows an operating speed of 700-800 M Hz and a transmission rate of 700 M bytes. If this is compared with other kinds of DRAM, the EDO (extended data-out) DRAM shows a transmission rate of 80M bytes, and the synchronous DRAM shows a transmission rate of 133M bytes, while the SGRAM (synchronous graphic RAM) shows a transmission rate of 400M bytes. Thus it is seen that the RAMBUS DRAM gives a large capacity transmission rate.
The RAMBUS DRAM has the prerequisite that it has to transmit a largest amount of data within a shortest period of time. If a large amount of data is to be transmitted at a high speed, the designed impedance of the component has to be matched with the designed impedance of the printed circuit board. If they are not matched to each other, then the signals are reflected, and time delay occurs in transmitting the signals, with the result that an accurate transmission is not realized, thereby making it impossible to carry out a high speed transmission. Accordingly, in manufacturing a printed circuit board for use in a communication equipment or the like, it is an important matter that the impedance of the circuit board be exactly controlled.
FIG. 1 illustrates the structure of the conventional multi-layer printed circuit board which is used for a high speed processing device such as RAMBUS DRAM. As shown in the drawing, the printed circuit board consists of 8 layers. As shown in the drawing, in the structure of this conventional printed circuit board, a first copper clad laminate 1 has a copper foil on each of the both faces, and the copper foils are etched to form circuits 3 and 5. A first adhesive layer 7 is disposed on the top face of the first copper clad laminate 1, and a second adhesive layer 9 is disposed on the bottom face of the laminate 1. The first adhesive layer 7 and the second adhesive layer 9 are made of prepreg. Then a second copper clad laminate 11 and a third copper clad laminate 15 are stacked on the top face and on the bottom face of the above structure, and then, a pressure is applied, so that the second copper clad laminate 11 and a third copper clad laminate 15 can be attached onto the first copper clad laminate 1.
Like the first copper clad laminate 1, the second and third copper clad laninate 11 and 15 are made to respectively have circuits 12, 13, 16 and 17 on both faces of them, which are formed by etching the copper foils of the both faces of them.
On the outer faces of the second and third copper clad laminates 11 and 15, there are respectively stacked a third adhesive layer 20 and a fourth adhesive layer 24 with a copper foil attached on each of them, and then, a pressure is applied on them, so that the copper foils would be attached onto the third and fourth adhesive layers 20 and 24. These copper foils are also etched to form circuits 21 and 25 on the third and fourth adhesive layers 20 and 24.
In the above printed circuit board, when the copper foils are attached on the insulating sheets, the copper foils and the insulating sheets are pressed together, thereby manufacturing the circuit board. Generally, a plurality of the printed circuit boards are manufactured in a single panel. At the most, 24 printed circuit boards are manufactured in a single panel. The pressing is carried out with a plurality of the panels stacked together. That is, under a vacuum within a vacuum chamber, the stacked panels are heated and pressed together.
Each of the circuit board panels has an area much larger than that of the printed circuit board. The prepreg which constitutes the adhesive layers is made of a glass fiber and an epoxy resin, and therefore, has a flow property. Accordingly, when the copper foils are attached onto the circuit board panels by applying a pressure, even if a uniform pressure is applied, a thickness difference occurs between the central portion and the peripheral portions due to the flowing property of the prepreg. This thickness difference remains after the manufacture of the printed circuit boards are completed. Accordingly, when the copper foils are etched to form circuits, insulating distance differences are formed between the copper foils, with the result that the impedance control for the printed circuit board becomes difficult.
The present invention is intended to overcome the above described disadvantages of the conventional technique.
Therefore it is an object of the present invention to provide a printed circuit board for a high speed processing, in which a plurality of both-face copper clad laminates with circuits formed thereon, and a plurality of clad laminates with circuits not formed thereon are alternately stacked together, whereby, when applying a pressure to attach copper foils, impedance defects due to the non-uniformity in the inter-layer insulating distances are eliminated.
It is another object of the present invention to provide a printed circuit board for a high speed processing, in which on both of the outermost faces of a structure consisting of copper clad laminate with adhesive layers interposed therebetween, there are formed thinner copper clad laminates, whereby, when pressing the stacked structure, impedance defects due to the non-uniformity in the inter-layer insulating distances are eliminated.
In achieving the above objects, the printed circuit board for a high speed processing according to the present invention includes: a plurality of copper clad laminates with copper foils attached on both faces thereof and with circuit formed at least on one faces thereof by etching; a plurality of copper non-clad laminates with copper foils on neither of their faces; and a plurality of adhesive means such as adhesive layers spread on faces of the copper non-clad laminates.
On outermost ones of the copper clad laminates, there are disposed either adhesive layers with circuits formed thereon, or there are disposed both-face copper clad laminates with circuits formed thereon.
Generally, the adhesive layers of the adhesive means are thinner than the both-face copper clad laminates, and therefore, when pressing the laminate structure, the thickness deviations can be decreased, thereby preventing impedance defects.
In another aspect of the present invention, the printed circuit board for a high speed processing according to the present invention includes: a plurality of copper clad laminates with copper foils attached on both faces thereof, with circuit formed at least on one faces thereof, and with adhesive layers interposed between the copper clad laminates; a pair of thinner adhesive layers spread on outermost faces of the copper clad laminates; and a pair of one-face copper clad laminates with circuits formed on copper foils and attached on the thinner adhesive layers.
Accordingly, compared with the general printed circuit board in which the circuits are formed directly on the adhesive layers, the thickness of the outermost adhesive layers is reduced, and therefore, the thickness deviations can be decreased when pressing the copper clad laminate structure.